This invention relates to whip antennas and more particularly to a circuit and method for accurately adjusting and aligning the whip antenna matching base unit to maintain proper and constant values of both the admittance measured across the matching base unit and the input impedance of the antenna.
Whip antennas are well known for use as vehicular antennas for ground tactical VHF-FM radio sets. One such typical antenna is shown in FIG. 1. It comprises an upper dipole element 10, usually a straight conductor, and a lower coaxial dipole element 12 terminated by a matching base unit 14 which includes a helical coaxial sleeve choke 16 and selective shunt reactances 18. The whip antenna of FIG. 1 is fed at point A of the matching base unit 14 and the coaxial dipole element 12 is connected from point B to ground through selective shunt reactances 18 as shown. The frequency range of operation for the communication equipment usually requires fixed tuning over several discrete bands to cover the entire communication range. For example, a system may require an operational range of 30 to 76 MHz which may be divided into ten discrete frequency ranges. It is the function of the matching base unit 14 to maintain the proper input matching impedance at antenna feed point A, and also the proper admittance measured from B to ground through one of the reactances 18 for each of the preselected frequency ranges. Heretofore, when such matching base units were manufactured for use with a whip antenna, the combined matching base unit 14 and antenna dipoles 10 and 12 were utilized to adjust the impedance at antenna feed point A of the matching base unit 14 in accordance with a calibrated standard. The tuning adjustments of the shunt reactances 18 were made so that the impedance at feed point A equalled the calibrated value for each discrete frequency range of operation. However, it was found that this procedure proved to be unsatisfactory since it resulted in widely varying values for both admittance and impedance during actual operation of the antenna over the entire frequency range. Moreover, the chamber in which the combined antenna and matching base unit was placed to provide such impedance measurements had to be rather long to include both the antenna and the matching base unit. Such a chamber proved to be rather unwieldy and expensive to make.